RFR: 8157605 Can't set both CONCURRENCY and EXTRA_JTREG_OPTIONS when running tests

[Staffan Larsen]

This does not work: 

$ cd test 
$ make jtreg_tests CONCURRENCY=10 EXTRA_JTREG_OPTIONS=-k:\!headful 

I can specify either CONCURRENCY or EXTRA_JTREG_OPTIONS, but not both.

Please review the fix below:

bug: https://bugs.openjdk.java.net/browse/JDK-8157605 <https://bugs.openjdk.java.net/browse/JDK-8157605>
webrev: http://cr.openjdk.java.net/~sla/8157605/webrev.00/ <http://cr.openjdk.java.net/~sla/8157605/webrev.00/>

Thanks,
/Staffan